..BEIRUT—As new details emerged about twin suicide bombings near the Iranian Embassy here, Lebanese officials described an outburst of violence that reveals the resurgence of al Qaeda-inspired groups in their country, a toxic byproduct of the Syrian war…….

U.S. officials believe members of Hezbollah, the militant group backed by Iran, are smuggling advanced guided-missile systems into Lebanon from Syria piece by piece to evade a secretive Israeli air campaign designed to stop them.

The moves illustrate how both Hezbollah and Israel are using Syria's civil war as cover for what increasingly is seen as a complex and high-stakes race to prepare for another potential conflict—their own—in ways that could alter the region's military balance.

Some components of a powerful antiship missile system have already been moved to Lebanon, according to previously undisclosed intelligence, while other systems that could target Israeli aircraft, ships and bases are being stored in expanded weapons depots under Hezbollah control in Syria, say current and former U.S. officials.

Such guided weapons would be a major step up from the "dumb" rockets and missiles Hezbollah now has stockpiled, and could sharply increase the group's ability to deter Israel in any potential new battle, officials say.

The movements appear to serve two purposes.

Iran wants to upgrade Hezbollah's arsenal to deter future Israeli strikes—either on Lebanon or on Iran's nuclear program, U.S. and Israeli officials say. In addition, these officials said they believe the transfers were meant to induce Hezbollah to commit to protect Syrian President Bashar al-Assad as well as supply lines used by both his regime and Hezbollah.

Israel struck inside Syria at least five times in 2013, seeking to take out systems bound for Hezbollah without provoking a direct confrontation.

U.S. and Israeli officials say the airstrikes have stopped shipments of ground-to-air SA-17 antiaircraft weapons and ground-to-ground Fateh-110 rockets to Hezbollah locations in Lebanon. Some originated from Iran, others from Syria itself.

Nonetheless, as many as 12 antiship guided-missile systems may now be in Hezbollah's possession inside Syria, according to U.S. officials briefed on the intelligence. Israel targeted those Russian-made systems in July and again in October with mixed results, according to U.S. damage assessments.

The U.S. believes Hezbollah has smuggled at least some components from those systems into Lebanon within the past year, including supersonic Yakhont rockets, but that it doesn't yet have all the parts needed there. "To make it lethal, a system needs to be complete," said a senior defense official.

Hezbollah already has around 100,000 rockets, according to Israeli intelligence estimates, but those are primarily unguided weapons that are less accurate. Its longer-range rockets are spread across Lebanon, meaning Israel's next air campaign—should one come—would have to be broad, Israeli officials have told their U.S. counterparts, according to American officials in the meetings.

Hezbollah's possession of guided-missile systems would make such an air campaign far riskier.

Current and former U.S. officials say Iran's elite Quds Force has been directly overseeing the shipments to Hezbollah warehouses in Syria. These officials say some of the guided missiles would allow Hezbollah to defend its strongholds in Lebanon, including Beirut, and attack Israeli planes and ground targets from regime-controlled territory in Syria.

Israel's Iron Dome missile-defense system can intercept and destroy short-range rockets. Its Arrow missile-defense system can intercept the sort of long-range ballistic missiles Iran possesses. A third system the Israelis are developing to deal with midrange guided missiles, called David's Sling, won't be operational until 2015 at the earliest.

Israeli officials say they are content for now to watch enemies No. 1 and No. 2—Hezbollah and Iran on one side, al Qaeda on the other—kill each other next door. U.S. intelligence agencies believe that Mr. Assad can hold on to a rump state bordering Lebanon and the Mediterranean for the foreseeable future, but won't be able to retake the entire country, U.S. officials say.

"It is arguably in Israel's interest to exploit the chaos without becoming embroiled in it," said Steven Simon, executive director of the International Institute for Strategic Studies in Washington and a former senior Obama administration official.

Israeli leaders made clear early on in the Syrian conflict that any transfers of advanced missile systems or chemical arms to its enemies would cross Israel's "red line."

Both the advanced missile and chemical programs are overseen by the same elite Syrian military-research center, which has close ties to Hezbollah, according to current and former U.S. officials. Syria agreed last year to give up its chemical weapons, a process that has yet to be completed.

The Organization for the Prohibition of Chemicals Weapons and the United Nations said last weekend that they would miss a Dec. 31 deadline for removing the most dangerous weapons because of volatile security conditions and logistical challenges. No new deadline has been set; the situation will be reassessed at a meeting of the OPCW's executive board on Jan. 8.

Since fighting a monthlong war with Hezbollah in 2006, Israel has ramped up an eavesdropping network to tap communications among Hezbollah, Iranian and Syrian regime figures to detect arms shipments, officials said.

Israeli officials said alarm bells sounded in late 2012 over a push by Iran to upgrade Hezbollah's arsenal with advanced guided-missile systems.

U.S. and Israeli spy agencies received intelligence that Iranian leaders, including the commander of the Quds Force, Qasem Soleimani, were increasingly concerned that the Assad regime was in danger of being overrun by rebels.

That meant Iran's window might have been closing to supply advanced weaponry to Hezbollah, Gen. Soleimani argued, according to officials briefed on the intelligence. From Tehran's perspective, Hezbollah's rockets were their first line of defense against an Israeli strike.

Current and former U.S. officials said the Assad regime also saw the weapons transfers as a way to fortify its relationship with Hezbollah, which it relies upon for survival.

Senior Israeli air force generals pushed for action to block the transfers, said Israelis familiar with the security deliberations.

To take out these systems without crossing into Syrian airspace, commanders directed Israeli pilots to perform a "lofting" maneuver designed to extend how far their bombs would travel, said U.S. officials briefed on the operations.

With a burst of speed and altitude, the pilots fling their GPS-guided bombs from ejector racks in a sweeping arc into Syria. In each case, the targets would have to be stationary, the officials said.

The first strike, on Jan. 30, targeted a shipment of Russian-made SA-17 antiaircraft missiles, U.S. officials said.

In early May, the Israelis tracked a plane they believed was carrying advanced Fateh-110 rockets from Iran to the Damascus International Airport, according to the U.S. officials briefed on the operations. Israel struck starting on May 2, from Lebanese air space.

That same month, Israeli and U.S. intelligence agencies began to track the antiship systems, whose Yakhont missiles can target warships with precision from over the horizon.

On July 5, Israel targeted some of the Yakhonts at a Syrian base outside the coastal city of Latakia. Afterward, Israeli and U.S. spy satellites saw something unexpected. Ground forces destroyed military equipment at the bombing site to try to trick Israel into believing it had successfully taken out the launchers, officials briefed on the intelligence say.

A U.S. damage assessment concluded that Israel had taken out only part of its target, and that the Yakhont missiles and launchers appeared to have been moved out of the line of fire. On Oct. 30 Israel targeted them again, U.S. officials said.

Israeli officials have told U.S. counterparts that the strikes damaged some Yakhont components, while others are stuck in warehouses in Syria.

"We don't think they have all the components in Lebanon to have a complete system," said a senior U.S. defense official.

But U.S. officials said they don't know the fate of all of the systems, and that they are concerned Hezbollah will bring more components into Lebanon.

Officials say supply lines for Hezbollah and the Assad regime have become increasingly intertwined, making it harder to distinguish between shipments bound for the Lebanese group and the regime. Israeli Air Force officials have told their American counterparts that commanders have aborted several planned airstrikes because of concerns about causing unintended damage.

U.S. defense officials said they believe Hezbollah has tried to throw off Israel's high-tech hunt by switching off and on communications and power networks along the border.

"Hezbollah is pretty damn good," said a senior U.S. official. "And they are patient."

New claims of poison-gas attacks by the Damascus regime are falling on deaf ears.

By DANIEL NISMAN

April 10, 2014 3:20 p.m. ET

Nearly eight months after the Assad regime gassed more than 1,400 people to death in Eastern Ghouta near Damascus, claims of chemical attacks in Syria's capital are once again falling on deaf ears in the international community.

On March 28, the rebels' Damascus Military Council reported a nerve-gas attack in the capital's Harasta area the previous night. These reports were echoed by several generally credible opposition news outlets. The number of reported casualties ranged from several dozen injured, to up to three dead from asphyxiation. Days later, on April 3, the opposition's Local Coordination Committees reported another chemical attack in the Jobar district, just as Bashar al-Assad's forces commenced an anticipated offensive to wrest the area from rebel hands.

Videos claiming to depict alleged deaths from the Harasta attack, and at least one injury from the Jobar attack, have since been circulating on social media. The Syrian opposition sent a letter to the United Nations demanding an inquiry, citing what they said to be credible evidence that chemical weapons had been used, to no avail. Most disconcertingly, an Israeli official told the Haaretz newspaper on April 7 that Jerusalem had "strong evidence pointing to the use of chemical materials in the Harasta neighborhood of eastern Damascus on March 27."

While the evidence is lacking to fully confirm whether chemical weapons were used in recent days, several conclusions can be drawn from the accusations themselves. The most worrying conclusion is certainly being drawn by the Assad regime—that of the international community's lack of interest to call for investigations in Harasta and Jobar.

The U.N.'s Organization for the Prohibition of Chemical Weapons (OPCW) has an established presence in Syria to carry out the September 2013 disarmament agreement sponsored by Russia and the United States. Whether an inspection falls under the OPCW mission's mandate or not, how the Assad regime reacts to demands to allow access to Jobar and Harasta would serve as an indicator of its culpability.

There is no credible evidence to suggest that rebel groups in the Damascus area have acquired the materials or know-how to mount chemical weapons on conventional artillery pieces in their possession. It can therefore be concluded that unless the rebels theatrically fabricated the effects of a chemical attack, the Assad regime was likely responsible for carrying them out. Notably, on March 25, Syria's U.N. Ambassador Bashar Ja'afari distributed a letter specifically warning that rebels would use chemical weapons in Jobar in order to blame the government. But if any party in the conflict would be prone to such conspiracy, it would be the Assad regime, whose decades of tutelage under the Russian KGB made their Mukhabarat (secret service) frighteningly efficient at false-flag tactics meant to smear the opposition.

Even with the majority of its chemical production and mixing facilities destroyed by the OPCW, and more than half of its chemical agents shipped out of the country, the Assad regime still has the ability to carry out localized attacks. Remember that last year, the Assad regime was accused of launching very limited attacks to dislodge rebels from frontline positions in major cities. These attacks, which employed agents such as sarin or mustard (or variants of the two), were carried out on a small enough scale to avoid drawing much international attention, while simultaneously deterring local populations from hosting rebel positions. The attacks were carried out using crudely modified short-range artillery rounds used by rebels, which would have allowed the Assad regime to blame the Syrian opposition and confuse the international community.

Between March and August 2013, the Assad regime was blamed for carrying out anywhere from six to eight of these attacks in Damascus, Aleppo, and Homs. Only when the large-scale attack in Eastern Ghouta on Aug. 21 produced overwhelming and undeniable evidence of the Assad regime's use of sarin, did the international community mobilize. As evidenced by leaked cables between Iran and Hezbollah regarding the attack, even the Assad regime and its allies understood they had pushed their luck too far.

Fast-forward to April 2014 and it is quite reasonable to assume that Assad believes he may once again have a free hand to use the world's most deadly weapons against his own people. The American naval destroyers that threatened to send hundreds of tomahawk missiles into Syria last September have long since departed the Mediterranean, and the eyes of the world's policy makers, including those in Moscow and Washington, are intently focused on the crisis in Ukraine. Despite missing several crucial deadlines in the chemical-weapons disarmament process, the U.S. has been careful to avoid accusing the Assad regime of specifically violating its international agreements, lest Washington be forced to resort to an unpopular military intervention.

With tweets, blogs, and videos of gassed Syrians resurfacing online, the same voices who vowed "never again" on the opinion pages and podiums of Washington, London, and Paris in August 2013 are now eerily silent. The international community must demonstrate its willingness to investigate every accusation of chemical-weapons usage. If not, Assad will interpret the world's silence as permission to use these most brutal weapons in the same escalatory fashion that eventually killed hundreds in Eastern Ghouta.

Bring in the International Criminal Court to investigate atrocities by all sides in the conflict.

By LAURENT FABIUS

Updated May 21, 2014 7:03 p.m. ET

The members of the United Nations Security Council must unite to bring the Syrian tragedy before the International Criminal Court.

The Syrian crisis is the most severe humanitarian crisis since the Rwandan genocide in 1994. In three years, more than 150,000 people have lost their lives, including 10,000 children, while 6.5 million civilians have been internally displaced and almost three million have sought refuge in neighboring countries.

Even war has rules. In Syria, these rules are flouted on a daily basis. Chemical weapons are being used; they killed 1,400 civilians in a single night on Aug. 21 last year. Barrel bombs have been used against schools. Attacks are committed against thousands of innocent civilians. Sexual violence against women is used as a weapon of war. Torture is used on detainees in the tens of thousands. International law characterizes these atrocities as war crimes and as crimes against humanity. If a scale of horror existed, these would be the most serious crimes.

As of today, those in Syria responsible for these crimes are not subject to prosecution. They are not tried, they are not sentenced. They continue on with full impunity.

But holding the perpetrators of war crimes and crimes against humanity accountable for their actions is a way of obtaining justice for the victims. It is also a deterrent to those who continue to commit such actions. Sooner or later, they will be judged.

An institution exists that is capable of investigating these crimes and punishing those who commit them: the International Criminal Court (ICC). That is why France has decided to submit a draft resolution to the U.N. Security Council referring the situation in Syria to the ICC.

Syria did not ratify the founding statute of the ICC and therefore does not recognize the court's jurisdiction over its territory. But the Security Council has the power to involve the ICC by adopting a resolution that refers the situation in Syria to the court and authorizes its prosecutor to conduct investigations and issue arrest warrants. The Security Council has already used this right twice in the past decade: to punish atrocities committed in Darfur and Libya. Now is the time to use this option for the Syrian tragedy.

Is the resolution likely to be adopted, given that previous efforts on Syria have been vetoed in the Security Council?

The resolution that we're proposing, with the support of a number of countries, is different: It targets all crimes committed in Syria, regardless of the perpetrators, and refers the matter to the ICC so that it can investigate the tragedy that has been unfolding in Syria since March 2011. The Syrian regime may have blood on its hands, but this resolution will not overlook the crimes committed by other groups.

The resolution focuses on the law. It is not a political resolution. It responds to the moral and political obligation to combat impunity. It is based on the responsibility to punish those who flout the elementary principles of humanity.

This is therefore a text that will bring people together. When the resolution is presented for a vote, the question each of the Security Council's 15 member states must answer will be simple: Am I for or against a system of justice charged with punishing those who, in the Syrian crisis, are responsible for crimes against humanity and war crimes?

As we examine this question, we must each assume our responsibilities, in the eyes of history and before the community of nations.

The United Nations was founded in 1945 precisely to oppose barbarity with the force of law. This fundamental principle cannot have been forgotten since then. That's why, unless some member states want to protect heinous crimes, it makes sense that another veto can be avoided and that the Security Council members can come to an agreement. We must finally be able to refer the Syrian matter to the International Criminal Court.

Mr. Fabius is the French minister of foreign affairs and international development.

Islamic State, or ISIS, Gained Momentum Early On From Calculated Decision by Syrian President Bashar al-Assad to Go Easy on It

By MARIA ABI-HABIB
Aug. 22, 2014 4:35 p.m. ET

Militant fighters from the Islamic State paraded in late June through Syria's northern Raqqa province, which the group has turned into a stronghold and base of operations. Reuters
The Islamic State, which metastasized from a group of militants seeking to overthrow the Syrian government into a marauding army gobbling up chunks of the Middle East, gained momentum early on from a calculated decision by Syrian President Bashar al-Assad to go easy on it, according to people close to the regime.

Earlier in the three-year-old Syrian uprising, Mr. Assad decided to mostly avoid fighting the Islamic State to enable it to cannibalize the more secular rebel group supported by the West, the Free Syrian Army, said Izzat Shahbandar, an Assad ally and former Iraqi lawmaker who was Baghdad's liaison to Damascus. The goal, he said, was to force the world to choose between the regime and extremists.

"When the Syrian army is not fighting the Islamic State, this makes the group stronger," said Mr. Shahbandar, a close aide to former Iraqi Prime Minister Nouri al-Maliki, who said Mr. Assad described the strategy to him personally during a visit in May to Damascus. "And sometimes, the army gives them a safe path to allow the Islamic State to attack the FSA and seize their weapons."

"It's a strategy to eliminate the FSA and have the two main players face each other in Syria: Assad and the Islamic State," said Mr. Shahbandar. "And now [Damascus] is asking the world to help, and the world can't say no."

The Islamic State, also known by the acronyms ISIS and ISIL, has emerged recently as a major threat to the entire region and beyond. Its seizure of territory in neighboring Iraq triggered American airstrikes, and its execution this week of kidnapped American journalist James Foley prompted President Barack Obama to vow to continue the U.S. air war against the group in Iraq and to relentlessly pursue the killers. General Martin Dempsey, the chairman of the Joint Chiefs of Staff, said the group can't be defeated without choking off its operations in Syria.

This account of how the Islamic State benefited from the complex three-way civil war in Syria between the government, the largely secular, moderate rebels and the hard-core Islamist groups was pieced together from interviews with Syrian rebel commanders and opposition figures, Iraqi government officials and Western diplomats, as well as al Qaeda documents seized by the U.S. military in Iraq.

The Assad regime now appears to be shifting away from its early reluctance to engage the group.

In June, Syria launched airstrikes on the group's headquarters in Raqqa in northern Syria, the first large-scale offensive against the militant group since it rose to power a year ago. This week, Syria flew more than three dozen sorties on Raqqa, its biggest assault on the group yet.

The Syrian ambassador to Lebanon, Ali Abdel-Karim Ali, denied that Damascus supported the Islamic State early on and praised his government's battlefield response to the group, pointing to dozens of recent strikes on the group's headquarters.

"Our priorities changed as these groups emerged," Mr. Ali said in an interview at his office. "Last month it was protecting Damascus, for example. Today it is Raqqa."

Speaking of the Islamic State aggression that has decimated the more secular FSA, he said: "When these groups clashed, the Syrian government benefited. When you have so many enemies and they clash with each other, you must take advantage of it. You step back, see who is left and finish them off."

Mr. Shahbandar said the Islamic State's recent success forced the Syrian government and its Iranian allies to ramp up their military assaults, hoping the West will throw its weight behind Damascus and Tehran to defeat the extremists. Such cooperation would put the U.S. and its regional allies such as Saudi Arabia in an uncomfortable position, after years of supporting the FSA and demanding that Mr. Assad step down.

There are some signs that the opposing sides might be willing to work together. In Iraq, the U.S. began arming Kurdish Peshmerga forces this month, while the Iranians sent advisers.

The Syrian government facilitated the predecessor to the Islamic State—al Qaeda in Iraq—when that group's primary target was U.S. troops then in the country.

In 2007, U.S. military forces raided an al Qaeda training camp in Sinjar, northern Iraq. They uncovered a trove of documents outlining Damascus's support to the extremists, according to the U.S. Military Academy at West Point, which publicly released the records. The Sinjar records detailed the flow of extremists from across the Middle East to the Damascus airport.

Syrian intelligence agents detained the fighters as they landed in the capital, holding them at the Sadnaya military prison on the city's outskirts. If deemed a threat to the country, they would remain imprisoned, the records indicate. But if their intentions were solely to fight U.S. troops in Iraq, Syrian intelligence would facilitate their flow across the border, the records show. Making that journey were many Saudis and Libyans—the same nationalities that today bolster the ranks of the Islamic State.

Mr. Maliki's former spokesman, Ali Aldabbagh, said in an interview that he attended heated meetings in Damascus during which Baghdad asked Mr. Assad to stop the flow of al Qaeda militants across the border. He said Syria brushed off the requests.

"The Assad regime played a key role in ISIL's rise," said U.S. State Department spokeswoman Marie Harf at a news conference earlier this month. "They allowed for a security situation where ISIL could grow in strength. The Syrian regime fostered the growth of terrorist networks. They facilitated the flow of al Qaeda foreign fighters in…Iraq."

The Assad regime denies providing any support to the groups.

By the time the U.S. military withdrew from Iraq in December 2011, the militant group was nearly decimated. It regrouped in northeast Syria as the revolution was becoming a civil war. It was led by a charismatic figure from Samarra, Iraq, who goes by the name of Abu Bakr al-Baghdadi.

In May 2011, after the first protests broke out in Syria, the Syrian government released from the Sadnaya military prison some of its most high-value detainees imprisoned for terrorism, the first in a series of general amnesties. At least nine went on to lead extremist groups in Syria, and four currently serve the Islamic State, statements from the extremist groups and interviews with other rebels show.

Mr. Ali, the Syrian ambassador to Lebanon, said Damascus had released only common criminals in the amnesties, who were then offered money by extremist groups to fight against the government.

"When Syria released these people, they hadn't committed terrorist crimes," he said. "They were just criminals. In 2011, there were calls for freedom and accusations that Damascus was imprisoning people, so we hosted several amnesties [to demonstrate] our goodwill."

Bassam Barabandi, a diplomat in Syria's foreign ministry at the time who has since defected, offered a different explanation. "The fear of a continued, peaceful revolution is why these Islamists were released," he said. "The reasoning behind the jihadists, for Assad and the regime, is that they are the alternative to the peaceful revolution. They are organized with the doctrine of jihad and the West is afraid of them."

The U.S. has been reluctant to supply arms to the moderate rebels for fear that the weaponry would wind up in the hands of extremists.

By the start of 2012, radical groups were entrenched in the Syrian uprising, with al Nusra Front, al Qaeda's Syrian arm, the biggest player. Last year, Nusra split over an ideological and leadership struggle. Most of the group's foreign fighters formed what was then known as Islamic State of Iraq and al-Sham, or ISIS, since renamed Islamic State.

The split between Nusra and the Islamic State created a fissure among al Qaeda supporters. The Islamic State presented itself as truer to al Qaeda's past, with its more radical social codes, and was more focused than its predecessor in creating a caliphate, or Islamic empire.

The Islamic State militants despised the FSA and its largely secular rebels, denouncing them as nonbelievers. By last summer, the Islamic State began grabbing territory the FSA had captured from the regime. In September, the Islamic State defeated the FSA's Northern Storm Brigade in Azaz, a border outpost between Aleppo province and Turkey. The Islamic State quickly imposed its hard-line version of Islam, forbidding smoking, enforcing the segregation of the sexes and conservative dress.

The Islamic State continued to take territory and impose its social codes on more of Syria, growing more ruthless over time. In January, disparate rebel factions united to turn their guns on Islamic State fighters, while angry civilians simultaneously rose up against the group. The FSA drove the Islamic State from its strongholds across Syria.

Shifting alliances between various rebel groups made the situation murky.

In the northern city of Raqqa, Islamic State fighters were ensconced in three municipal buildings by mid-January, surrounded by rebels from the FSA and Islamic Front, a coalition of religious rebel groups. The Islamist militia Ahrar al-Sham, fighting alongside the FSA, posed the biggest threat, and Islamic State fighters appeared ready to surrender to that group.

"They got on the loudspeakers and said, 'We are your Muslim brothers. Don't kill us. Let us withdraw peacefully with our weapons,' " said Mohammed Abu Seif, an FSA rebel in Raqqa who was present at the standoff.

FSA fighters said their leaders wanted to continue the attack. They were prepared to kill the Islamic State militants, said Mr. Abu Seif and several other rebels involved in the fighting.

But Ahrar al-Sham wavered, they said, taking pity on their Muslim brethren. FSA fighters pressed on, hoping to wipe out the Islamic State and restore the secular roots of their revolution, according to Mr. Abu Seif and the other rebels.

But by the fourth day, Ahrar al-Sham started to withdraw from Raqqa. Rebels say a previously unreported deal was cut for Ahrar al-Sham and the Islamic State to swap territory. The Islamic State agreed to withdraw from Aleppo and Azaz, a border crossing with Turkey. In exchange, Ahrar al Sham would withdraw from Raqqa and Tal Abyad, another border town.

The FSA found themselves surrounded in the Raqqa suburbs by thousands of Islamic State fighters who were retreating from FSA advances elsewhere. On the eighth day, the FSA and its affiliates retreated, leaving Raqqa to the Islamic State.

By the spring, the Islamic State had used what amounted to a sanctuary in Raqqa to rejuvenate its ranks. With Raqqa as its base and headquarters, the militants went back on the offensive, storming across Syria, while its branch in Iraq did the same just across the border.

A member loyal to the Islamic State in Raqqa, Syria, in June. Reuters
By June, Abu Bakr al-Baghdadi declared a caliphate and renamed ISIS the Islamic State, declaring nearly 12,000 square miles of contiguous territory across western Iraq and in Syria's north and east—an area the size of Belgium—a newly formed Islamic caliphate. The group now threatens the borders of Jordan, Saudi Arabia and Lebanon, where it briefly occupied a Lebanese border town this month.

Still, at times its actions appeared to help the Syrian government in its fight against the FSA. Aleppo, Syria's largest city, remained one of the few major strongholds of FSA resistance. Last month, the Islamic State quietly withdrew from the city's northeastern suburbs, clearing the way for Syrian government forces to stream in. Not a shot was fired. The gains enabled government forces to flank FSA rebels from three sides in Aleppo.

As FSA fighters struggle to hold off the regime, they also are fighting Islamic State militants in the countryside just north of Aleppo. Only 4 miles remain to fully encircle and besiege Aleppo. If FSA rebels lose the battle, it could spell the end of their revolution, rebels say.

Today, at a time when the FSA's ranks are thinning, new recruits from the Middle East and beyond are flocking to the Islamic State, crossing the Turkish border to settle themselves and sometimes their families in Raqqa. The group's fighters and core members are largely Syrians and Iraqis, but recruits are arriving from as far away as Europe and the U.S., say American intelligence officials. The U.K. chief of police in charge of counterterrorism estimated in June that 500 Britons alone have joined the group, although a member of British Parliament has said the number could be as high as 1,500.

At a recent U.S. intelligence briefing, American officials estimated the Islamic State's size to be about 10,000 before it took over Mosul, Iraq's second largest city, in June. European diplomats say the number may be as high as 20,000.

In June, after the Islamic State took over most of western Iraq and eastern Syria, controlling much of the border between the two countries, the Syrian regime began shifting its approach, striking Raqqa from the air. Since then, the Islamic State's appetite to attack the regime has grown, and it has assaulted government forces across Syria.

Iraqi officials say the strike on Raqqa may have been prompted by Baghdad's anger toward Damascus for allowing the Islamic State to rise to prominence in Syria, emboldening its Iraq branch.

Syrian civilians living in Raqqa and rebels said that unless the U.S. is willing to expand its military strikes against the Islamic State to include Syria, the group will continue to grow.